The zonally averaged circulation, temperature, and compositional structure of the lower thermosphere and variations with geomagnetic activity

A zonally averaged chemical‐dynamical model of the thermosphere is used to examine the effect of high‐latitude particle and Joule heating on the neutral composition, temperature, and winds at solstice for solar minimum conditions. The meridional circulation forced by solar heating alone is a summer‐to‐winter flow, with a winter enhancement in atomic oxygen. The high‐latitude heat sources drive mean circulation cells that reinforce the solar‐driven circulation in the summer hemisphere and oppose this circulation in the winter hemisphere. The changes in wind and temperature caused by the high‐latitude heat sources increase the relative concentration of N 2 and O 2 in the high‐latitude upper thermosphere and decrease the O concentration in the high‐latitude lower thermosphere. For prolonged moderate levels of geomagnetic activity the peak atomic oxygen density in the polar regions can decrease by factors of 2–3 from geomagnetic quiet conditions.